This invention relates to air filtering inside clean rooms.
Air filtering is critical in the manufacture of semiconductor devices. Tremendous efforts are made to eliminate contaminants from the semiconductor device manufacturing site, commonly referred to as a clean room. Clean room contaminants may be generally classified as either particulate or gas-phase. Common particulate contaminants include dust, lint and other debris. Examples of gas-phase contaminants, which are dimensionally 30,000-40,000 times smaller than particulate contaminants, include acid gases, base gases including ammonia and other reactive amines, volatile organic compounds (VOCs), and boron.
Until recently, most of the efforts in clean room design have focused on removing particulate contaminants, which were viewed as having the most impact on device yields and device performance. However, it has been discovered that gaseous contamination is an important limiting factor in the further reduction of device geometry and the improvement of device performance.
Attempts have been made to reduce gas-phase contamination by incorporating traditional rack and tray type gas-phase filters into existing cleanroom air handling systems. A rack and tray filter generally includes a perforated metal structure which contains loose sorbent particles. Because rack and tray filters tend to generate large quantities of particulate contamination(due, at least in part, to the vibration of air handling systems), additional particulate filters, preceding a final high-efficiency particulate air (HEPA) filter, are required downstream from the rack and tray system. These additional particulate filters require frequent servicing due to accumulation of particles in the filters.
One cleanroom design scheme to reduce gas-phase contamination involves isolating the process stations which generate particular contaminants from those processes which are sensitive to those contaminants (e.g., by constructing barriers between stations, or by increasing the distance separating individual processing stations). Such solutions have affected capital cost and work flow efficiency.
Boron is a particularly troublesome gas-phase contaminant which is found naturally in the air as boric acid, and is given off by borosilicate glass in the high efficiency particulate air (HEPA) filters commonly used in clean room air recirculating systems. Reduction of semiconductor device yields has been traced to relatively low levels of boron contamination which causes counter-doping of lightly doped n-type layers. Since the atmosphere and the HEPA filters are sources of boron contamination, it has been viewed as impractical to remove boron from the clean room air. Instead, process engineers accept boron contamination as inevitable and typically add an additional cleaning step (a buffered HF clean/etch) to reduce the level of boron on the wafer surface. Alternatively, process engineers may simply modify the required doping levels to compensate for the doping effect of boron.